The passenger or occupant seated in a vehicle and particularly a helicopter can be subjected to a combination of forces during a crash. If the occupant is appropriately restrained in a seat, the forces generally acting horizontally (i.e., x and y-axes) are typically considered survivable. However, the forces acting substantially vertically (i.e., z-axis) or along the spine of the occupant can produce significant injuries. Injuries to the spine and particularly to the lumbar region can potentially result in paraplegia or death. To mitigate such injuries, energy absorbing seats are generally used in helicopters, wherein the portion of the seat supporting the occupant is allowed to move or travel by the occupant's inertial loading during impact. The movement of the seat is referred to as stroking and its movement, or stroke, is resisted by the force/s applied by Energy Absorbers (EAs), elongating the stopping distance while absorbing crash energy and reducing the loads imposed on the occupant. The energy absorbers are made to absorb as much energy as possible at loads that are survivable but which stop the seat and occupant before contact with the floor of the vehicle. If the energy absorbing capacity of the EAs are exceeded, the seat and occupant will not stop stroking before reaching it stroking limits in which case it “bottoms out” imposing a sudden impact load on the occupant which can overstress the already stressed spine producing a facture.
The stroking of helicopter seats has been achieved in the past by EAs using a constant force-displacement (stroke) characteristic (See generally, Desjardins, S. P. “The Evolution of Energy Absorption Systems for Crashworthy Helicopter Seats,” Journal of the American Helicopter Society (2005). These EAs were called Fixed Load Energy Absorbers (FLEAs). The FLEA method attempts to protect the universe of occupants by providing energy absorbers that stroke at a determined by multiplying the mass of the stroking part of the seat plus the seat-supported mass of a reference occupant by a constant factor expressed in G's. The factor was established as producing a safe deceleration force for a 50th percentile male of the military occupant population. These energy absorbers when stroked completely will absorb the same amount of energy for all occupants regardless of weight since they stroke at the given design force. Consequently, the FLEA is most effective for an occupant having a weight approaching that of the reference occupant. However, the FLEA performance diminishes as the occupant's weight diverges from the weight of the reference occupant. For example, with energy absorbers designed for the 50th percentile occupant, a lighter occupant is generally exposed to greater deceleration than a heavier occupant, because the stroking force of the EA is sized for the mass of the 50th percentile reference occupant and not that of the lighter occupant. This means that the lighter occupant will stroke at a higher acceleration (G) than the heavier occupant. On the other hand, an occupant heavier than the 50th percentile weight can be substantially more at risk because his increased mass will produce stroking at lower G's which means that the seat must stroke further to absorb the additional energy produced by the increased mass of the heavier occupant. That force is generally less than is tolerable for the heavier occupant and the longer stroke results in increased risk of exceeding the available stroke and impacting the floor under the seat. The maximum forces created by impact with the floor or any fixed structure in the path of the seat while stroking can be substantial as it imposes a sudden impact load.
Moreover, the state-of-the-art has always been to minimize time and EA stroke to reach maximum load (EA force) or, in other words, to maximize the rate of onset. (As used herein, the word “force” is generally used to describe the force required to elongate or “stroke” the energy absorbers; whereas, the word “load” is used to describe the load in the spine which is contributing to the force being reacted by the energy absorbers.) The approach of maximizing the rate of onset was established to maximize the energy under the EA force-time curve. (Given a maximum stroke distance, the maximum energy that can be absorbed ((area under the curve of EA force vs stroke)) is produced by a rectangular shape.) This suddenly-applied impact loading approach, however, created dynamic overshoot, a phenomenon caused by the compressibility of the human occupant's spine.
Because the human spine is compressible, there is an inherent delay between when the EA begins to apply a force to decelerate the occupant and the time the occupant's spine has compressed to support the load. As with any damped spring-mass system, if the period of the applied load from the EA to the occupant's spine is too short, the loading of the occupant's spine will lag the input producing peak loads that exceed the applied force of the EA, leading to potential injury. This phenomenon is called dynamic overshoot. The prior art solution to the problem of dynamic overshoot, was simply to lower the entire force setting on the EAs to limit spinal loading to tolerable magnitudes. This resulted in a significant reduction in efficiency of the EA. Researchers also tried to solve the problem created by dynamic overshoot by reducing the force produced by the EAs after the initial onset of loading, creating a “notch” in the EA force versus stroke curve. The timing of this application, however, occurs too late in the event to solve the problem of dynamic overshoot as the occupant response lags initiation of loading.
To overcome the inability of a FLEA to be adjusted for different occupant weights, the Variable Load Energy Absorber (VLEA) was developed. The VLEA is essentially a FLEA in which the EA stroking force can be manually adjusted to a more appropriate level to account for the actual weight of the occupant, i.e. higher EA forces for heavier occupants and lower forces for the lighter occupants. The disadvantage common to both the FLEA and the VLEA is the dynamic overshoot during the initial portion of the deceleration pulse. The stroking force of the EAs must be set low enough to protect the occupant's spine during the initial onset of the pulse resulting in a reduced capacity to absorb energy later in the stroke after the dynamic overshoot has subsided. This is because once the force is set it remains constant at that level during the complete stroke producing spinal loads during the latter parts of the stroke that are significantly lower than would be tolerable.
To mitigate the effects of the dynamic overshoot, while increasing the efficiency of the EA, the Fixed Profile Energy Absorber (FPEA) was developed. The FPEA method provides a decelerating force on the occupant that varies with the seat stroke. It is important to note that the force varies with stroke, not with real time. The variation in force with stroke is produced by an EA mechanism that is designed to produce a specific force vs stroke characteristic that produces lower EA forces initially to limit the dynamic overshoot forces to the tolerable range followed by a higher force versus stroke characteristic later in the seat stroke. This results in a more efficient stroke as the forces decelerating the occupant are always closer to the occupant's tolerance level meaning that more energy is being absorbed at tolerable levels. Test data verifies the improved efficiency of the FPEA characteristic over the constant force characteristics used in all FLEAs and VPEAs. The FPEA, however, like the FLEA is not adjustable for occupant weight and, therefore, when stroked completely will absorb the same total energy irrespective of the occupant's weight.
The, FLEA, VLEA and FPEA methods all provide limited protection for a military seeking greater diversity in personnel. This diversity has resulted in a population that includes an increasing number of female soldiers. For at least this reason, the range of body size and the disparity of weight of the soldier in helicopters has increased. Further, a new generation of crashworthy technology including improved micro-electromechanical (MEMS) sensors and semiconductor electronic devices can provide greater speed and accuracy in determining an incipient crash. Employing new technology is necessary to provide optimal safety and survivability to occupants having a wide range of weight and size. The, FLEA, VLEA and FPEA methods are lacking for not employing the new generation of crashworthiness technology. The FLEA, VLEA and FPEA methods do not individually and automatically measure and correct the EA setting for the occupant's weight and the FLEA and FPEA cannot be adjusted for the occupant's weight even if the data were available. None provide any compensation to the forces needed to safely decelerate occupants over a broad assortment of crash environments.
VLEAs are provided with manual adjustments to enable an occupant to adjust the EA stroking force for occupant weight. The FPEA instead depends on the shape of the profile itself to protect the light occupant with the low EA force being applied over the beginning few inches of stroke followed by the next few inches at intermediate force to protect the bulk of the user population and with the remaining few inches at higher forces to protect the heavier occupants. The design of the FPEA, is quite inefficient as most of its available stroke is used in decelerating heavier occupants using much lower forces than are tolerable; however, it can be more efficient than the FLEA which is limited by a constant EA force that must be low enough to protect the lighter occupants which means it never operates at the optimum stroking force for the heavier occupants.
Other technologies attempt to solve the problems associated with limiting the forces imposed on the human to within tolerable magnitudes by measuring the response of the body to the loading and adjusting the EA forces in real time, as opposed to providing loading profiles that are predetermined and selectable either by the occupant prior to flight or by a vehicle-borne sensing and control system. One of the problems associated with systems using real time measurements to adjust the EA applied force is determining what parameter to measure for use in controlling the applied EA force. It is not possible to measure spinal loads in a living occupant and, consequently, a secondary measurement indicative of spinal load must be used. The usual approach for making this type of approximation is a measurement of seat accelerations; however, seat acceleration is not an accurate indicator of spinal load. Another problem associated with these types of systems is created by the very short time available to make the measurement, convert that information into a correction and then in making the correction in time to affect the immediate loading of the occupant. The total time of the Government-specified vertical crash pulse for helicopters is 0.043 to 0.087 seconds depending on seat location in the vehicle and these systems must make the corrections instantaneously during that period to correct an effect that occurred previously. It must also be remembered that spinal load is a result of a human body's response to previously applied loading and therefore lags the imposition of the causative loading. Since the correlation between those two parameters and spinal load has not been established, the technology has not been proven.
Hence, there is a need for a method to absorb a portion of the kinetic energy of any occupant of a crashing aircraft using humanly tolerable forces regardless of weight and as a function of the specific crash characteristics. Such a method would use the most efficient force vs stroke profile possible, eliminating the effects of dynamic overshoot, and accounting for the occupant's weight and decelerating the occupant using the lowest stroking forces possible while using the entire available stroke to absorb the necessary energy to preclude the stroking portion of the seat from contacting the floor of the vehicle. It would also provide a humanly survivable environment in the most severe crash possible. This would also mean eliminating the effects of dynamic overshoot while using a profile providing the maximum tolerable loading over the entire available seat stroke.